1. Field of the Invention
The present invention relates in general to video signal processing systems, and more particularly to an apparatus and a method for enhancing a transient edge of a video signal, in which a distinct picture of high quality can be obtained.
2. Description of the Prior Art
Conventionally, to obtain a distinct picture, there has been proposed an apparatus for enhancing a transient edge of a luminance signal or a color signal separated from a video signal. More particularly, the conventional enhancement apparatus is adapted to allow the user to visually feel the distinct picture, by dropping an overshoot and an undershoot of the edge as represented in FIG. 1. One example of this form of apparatus is shown in a block form in FIG. 2, herein.
As shown in FIG. 2, the conventional apparatus for enhancing the transient edge of the video signal comprises delays 1 and 2 for sequentially delaying the video signal inputted therein, amplifiers 3-5 for amplifying the original video signal and the video signals delayed by the delays 1 and 2, respectively, an adder 6 for adding output signals from the amplifiers 3-5, an attenuator 7 for adjusting an enhancement level of an output signal from the adder 6, a delay matching unit 8 for matching an output signal from the attenuator 7 with the output signal from the delay 1, and an adder 9 for adding an output signal from the delay matching unit 8 to the output signal from the attenuator 7 and outputting the resultant enhanced video signal.
The operation of the conventional enhancement apparatus with the above-mentioned construction will hereinafter be described.
Upon inputting the video signal, the amplifier 3 amplifies the inputted video signal and then applies the amplified video signal to the adder 8. Also, the inputted video signal is sequentially delayed by a predetermined time period by the delays 1 and 2. The video signals sequentially delayed by the delays 1 and 2 are amplified by the amplifies 4 and 5, respectively, and then applied to the adder 8.
Noticeably, the inputted video signal is enhanced vertically if the delay time of the delays 1 and 2 is a period of one line, while horizontally if the delay time is a period of one sample. Namely, since the overshoot and undershoot occur at the transient edge of the video signal differentiated by the delays 1 and 2 and the amplifiers 3-5, the edge enhancement is performed.
The enhancement level of the enhanced signal is adjusted by the attenuator 7, which then applies the enhanced signal of the adjusted enhancement level to the adder 9.
On the other hand, the delay matching unit 8 matches the output signal from the attenuator 7 with the output signal from the delay 1 and then applies the matched signal to the adder 9. As a result, the adder 9 adds the output signal from the delay matching unit 8 to the output signal from the attenuator 7 and outputs the resultant enhanced video signal.
For example, in a NTSC broadcasting system, the luminance signal has a band width of 4.2 MHz with its edge portion being 2 to 3.5 MHz. Therefore, the distinct picture is obtained by enhancing the frequency band component of the edge portion using the delays and amplifiers. By the way, various signal processing systems such as a VCR, a NTSC broadcasting television (TV) receiver, a S-VHS and etc. have different natural frequency characteristics of video signals as shown in FIGS. 3 and 4. For this reason, the enhancement on the transient edge must accurately be performed according to the different natural frequency characteristics.
However, the conventional enhancement apparatus has a disadvantage in that the Frequency region to be enhanced is fixed to 2 to 4 MHz. Namely, in the conventional enhancement apparatus, the enhancement on the transient edge is performed on the basis of the NTSC broadcasting TV having the band width of 4 MHz as shown in FIG. 3. For this reason, in the case where the conventional enhancement apparatus is applied to the VCR or S-VHS, a frequency band other than the edge portion is enhanced, resulting in an unnaturalness and a degradation in the picture quality.
In other words, when the enhancement apparatus for the NTSC broadcasting TV is applied to the VCR, a high frequency signal component is enhanced, thereby causing the output video signal to have a considerable amount of noise. Also, in the case where the enhancement apparatus for the NTSC broadcasting TV is applied to the S-VHS, the edge portion is enhanced together with a frequency component lower than that, resulting in the unnaturalness in the output video signal.
As shown in FIG. 4, since the video signals in the NTSC broadcasting TV, the VCR and the S-VHS have the respective limited band widths, the transient edges thereof are of widths increased as compared with an ideal signal and have a frequency difference therebetween.
Also, a relatively large overshoot occurs at a sharp transient edge, thereby causing the video signal to be enhanced together with a noise component. This results in a degradation in the picture quality. Moreover, the video picture is unnatural because only the start point (undershoot) and end point (overshoot) of the transient edge of the video signal are enhanced. Further, the distinct picture cannot be obtained since the width and slope of the transient edge of the video signal are fixed.